A Study On The Corrosion Behaviors And Plasma Electrolytic Oxidation Of Zirconium Alloys

Zirconium alloys are one of the most important structural materials in nuclearindustry, the study of the corrosion and protection of which has important significance.In this paper, the effect of Cu²⁺on corrosion behavior of zicaloy-4alloys was studiedby electrochemical methods, and the mechanism of pitting corrosion of zirconiumalloy caused by Cu²⁺was discussed. Then, the plasma electrolytic oxidation（PEO） ofzicaloy-4alloys and zicaloy-2alloys were studied in detail via DC or pulsed currentregimes. The microstructure, phase composition and corrosion resistance of the PEOcoatings with different processing parameters are investigated.The research shows that the Cu²⁺ions can shift the corrosion potential ofzircaloy-4alloys toward the positive direction and the corrosion current density alsoincreases, while the decreasing of the pitting resistance of zircaloy-4is the synergeticeffect of Cu²⁺and Cl-. PEO treatment can greatly enhance the pitting resistance ofzircaloy-4alloy.The PEO behaviors of zircaloy-4in Na₂SiO₃+KOH, Na₂SiO₃+Na₃PO₄+KOH,Na₂SiO₃+Na₄P₂O₇, Na₄P₂O₇+KOH electrolyte systems at DC current regime of200mA/cm2were studied in detail, and the influence of electrolyte components on thephase compositions of the coatings were analyzed. The results show that theimprovement of anti-corrosion properties of zircaloy-4alloys depends on the highquality coatings prepared by PEO technology. The PEO coatings acquired inNa₂SiO₃+Na₄P₂O₇, Na₄P₂O₇+KOH electrolyte systems are homogeneous and havegood corrosion resistance.Different PEO coatings were prepared on zicaloy-2alloys in alkaline silicateelectrolytes using a pulsed current regimes by changing the oxidation time, pulsefrequency, positive duty ratio and negative duty ratio. The results of potentiodynamicpolarization curves show that the PEO coatings treated at the oxidation times of10min, pulse frequency of1000Hz, positive duty ratio of20%and negative duty ratioof20%has the best corrosion resistance. The spark behaviors of zircaloy-2alloys insilicate system were investigated and from the point of view of the low thermalconductivity of zirconium oxide, the characteristic solidification structures formed inthe PEO coatings with silicate electrolytic was further discussed.The PEO treatment of zircaloy-2with pulsed current regime was also carried outin alkaline electrolyte with sodium aluminate, the microscopic morphology, phase compositions and the protection ability of the coatings were analyzed. The resultsdemonstrate that the thickness of the coating no longer increases when reaches itslimit, this phenomenon has relationship with the burning destruction of the coatingmaterials by the repeating sparks occurred at paticular locations of the coating.The phase compositions of the PEO coatings of zirconium alloys were alsostudied. The results show that monoclinic and tetragonal ZrO₂are contained in thecoatings under different processing parameters, however, the relative content of thetwo phases varies with different electrolyte systems and types of current regimes.